Sulfuric acid pickling bath



CRUSS REFERICE Dec. 10, 1968 I.. E. KovALsKl SULFURIC ACID PICKLING BATH Filed April 27, 1966 lh' VEN TOR Leonard E Kava/ski United States Patent Qflce 3,415,748 SULFURIC ACID PICKLING BATH Leonard E. Kovalski, Allentown, Pa., assignor to Bethlehem Steel Corporation, a corporation of elaware Filed Apr. 27, 1966, Ser. No. 545,712

Claims. (Cl. 25279.4)

ABSTRACT 0F THE DISCLOSURE In a method of removing mill scale from steel articles, a steel article is immersed in an aqueous solution contain- This invention relates to the pickling of ferrous metal surfaces, and more particularly to an improved composition for performing such pickling.

In the pickling of ferrous metals such as iron, carbon of the pickling bath.

Generally, scaled iron or carbon steel surfaces can be pickled without any great steels.

Another object is to provide a bath which will operate efficiently with a high dissolved iron content.

The accompanying drawing is a graph illustrating the amount of ferrous iron (Fe++) permissible in an operating bath of this invention.

tool and alloy constructional steels, in a bath composition of this invention, overcomes many of the problems encountered with known pickling Patented Dec. 10, 1968 usually high tolerance for dissolved iron.

While the foregoing pickling benefits are obtained over simultaneously or successively the same bath without any significant loss of bath efbath control is relatively simple,l as the bath 1s operable at rather wide temperature and composition ranges.

The bath is particularly applicable to batch pickling of billets, bars, angles, ats or wire.

e composition of the bath of this invention may range as follows:

Percent by weight Sulfuric acid H2804) 1-20 Inhibitor 0.01-0.1 Surfactant 0.03-02 Dissolved iron as Fe++ 0-15 Temperature 1 160-200 1 Degrees Fahrenheit.

according to ple which follows.

EXAMPLE 1 In the particular operation to be described, a mixture of alloy billets of varying size, ranging from 9 x 9 inches to 2.5 x 2.5 inches in crosssectional area and having a feet, with the mixture pickling bath for 2 hours, after which they were removed and rinsed.

The pickling treatment removed the heavy scale from the billets, the surfaces of which presented a clean bright appearance which could readily be inspected visually for surface defects.

A new bath, of the type just described, -can be operated with successive charges of scaled billets for a period of about 8 hours without additions to the bath. The temperature should be maintained, preferably, between 175 and 185 F. For most efficient operation, the acid concentration Vshould be above 3% by weight, although satisfactory operation is maintained with the acid as low as 1%.

If successive charges of billets each represent a weight approximately that of the example, the bath should be replenished at the end of about 8 hours. This can be done by adding 125-150 gallons of sulfuric acid (56 B.), about pounds of 1,3-dibutylthiourea and between 2.5 and 3 gallons of the surfactant (27% aqueous solution). The amount of make-up acid and other make-up ingredients to be added, at the end of each 8 hour period, can be determined accurately by analyzing the bath for free sulfuric acid and adding ingredients sucent to bring the bath up to its original stren-gth. The amount of inhibitor and surfactant to be added, with addition of sulfuric acid, is as follows: l

Inhibitor is added in the amount of 4-5.5 pounds for each 100 gallons of acid (100% HZSO) added.

Surfactant is added in the amount of 2-3 gallons (27% by weight aqueous solution) for each 100 gallons of acid (100% H2SO4) added.

Such analysis and additions can of course be made at any time during bath operation if it is desired to replenish the bath more Ifrequently than the 8hour interval mentioned.

Normally, a bath of this invention may be operated for from 7 to 8 days, and for 3 full turns per day, before being dumped to permit cleaning of sludge from the pickling tank. However, the bath can be operated satisfactorily for longer periods if the dissolved iron does not appreciably exceed by weight.

The types of alloy steels pickled in the above example included those designated as AISI 4027, 4118, 4620, 5130, 6150, 8740, 43201-1, 5120H, 411.40, 43L70, 01, 06, A2 and W1.

No foam blanket was used on the bath, fumes being conducted away from the bath surface by overhead exhaust.

Bath components can be varied over a wide range, as previously noted. For Example 1, the preferred range of bath composition is as follows:

Percent by weight Sulfuric acid (100% H2804) 2-8 Inhibitor 0.01-0.05 Surfactant 0.05-0.12 Dissolved iron as Fe++ 0-15 Temperature 1175-185 1 Degrees Fahrenheit.

The Abath of Example 1 was run for seven consecutive days, durin-g which time 930 tons of steel were pickled. Consumption of 56 B. sulfuric acid amounted to 3025 gallons, while 117 pounds of 1,3-dibutylthiourea and 53 gallons (27% aqueous solution) of sodium tetradecyl suffate were consumed. Dissolved iron reached 150 grams per liter on the fth day. This represented approximately 11% iron calculated as Fe++.

EXAMPLE 2 In another pickling operation, this one utilizing a pickling tank of 4000 gallons capacity, where carbon steel bars, an-gles, ats, etc., as well as alloy billets are ordinarily pickled, about 1000 gallons of water were introduced into the tank, followed by addition of 15 pounds of 1,3-dibutylthiourea, uniformly distributed. After adding 150 gallons of 66 B. sulfuric acid, an additional 1000 gallons of water were added to bring the bath volume to 2150 gallons. At this point, 4 gallons (26% aqueous solution) of surfactant (sodium tetradecyl sulfate) were added with uniform distribution throughout the bath. During each addition steam from spargers located in the tank walls was introduced into the bath to bring the bath up to temperature and to agitate the bath. To insure thorough distribution of -bath components, water additions were made from a hose. When the bath temperature reached -170 F., the volume was adjusted to 2500 gallons by adding more water. The pickling charge, in this case alloy steel billets, was introduced when the bath had acquired a temperature of about 160 F.

In this operation, a foam blanket was used as there was no exhaust hood over the pickling tank. In order further to control fuming, the bath temperature was maintained at between 160 and 170 F. To maintain a desirable pickling rate, at the lower temperature, the acid concentration of the starting bath was higher than that of Example 1.

The pickling time for each charge of between 12-15 tons of steel was about 1.5 hours. It was necessary to add l gallon of surfactant and 5 pounds of 1,3-dibutylthiourea after each 8 hour pickling period. Suicient acid was supplied to maintain the concentration at 7.9% by weight.

With a bath of the type used in Example 2, where it is desired to control fuming, the acid content, as noted above, should be somewhat higher than the preferred operating ran-ge for the bath of Example 1. Bath components for Example 2 may preferably range as follows:

Percent 'by weight With the higher acid content of this bath, the operating bath temperature should range between 160-170 F.

Alloy billets of the type described in Example 1 were pickled in the bath of Example 2 when the fresh bath was first used. Later, a mixture of alloy and carbon steel billets were pickled simultaneously. All articles pickled displayed a clean surface after 1.5 hours. It is necessary, in the case of billets, to remove the scale sufficiently completely to reveal any surface defects, so that any defects present can be removed by scarfing, etc. Improperly pickled billets often retain sufficient tenacious scale in localized parts to completely obscure the underlying surface defects.

While the baths of Examples 1 and 2 were each run for a one-week period before dumping, baths of the same composition described in each of the examples have been in use for a period of at least three months at the same installations, wherein the baths were renewed at the end of each Weekly period. The advantages of the 4baths heretofore mentioned were realized throughout the entire three month period.

In order to illustrate the advantages which can be expected from a pickling operation with a bath of this invention, a laboratory test run was made on alloy steel specimens. Weighed specimens of alloy steel billets with mill scale, measuring 4 by 4 by 4 inches were suspended in approximately 3.2 gallons of an aqueous solution containing 5.0-5.8% by weight of sulfuric acid, 0.05% by weight of 1,3-dibutylthiolr1rea and 0.10% by weight of 27% aqueous solution of sodium tetradecyl sulfate at -195 F. The solution, contained in a 51A gallon glass cylinder, was preheated by ceramic immersion heaters and agitated with a stirring motor. The billet specimens were at room temperature when introduced into the bath, and were allowed to remain in the bath until completely descaled. When descaled, the specimens were rinsed with cold water, blown dry with air, and reweighed. The bath was analyzed to determine acid consumption, ferrous iron increase, and volume loss.

A similar test was conducted in an aqueous bath containing uninhibited sulfuric acid in an amount of approximately 5.5% by Weight, as a control.

Results of the tests are give'n in the following table:

Loss ot Sulturic Sulfuric Increase Initial Descaling Weight Acid cid of lron Volume Reduction Test Billet Grade Weight Time of (100%) (100%) (Fett) Loss of in Descaling of Billet (min.) Billet in Bath Consumed in Bath Bath Time (lbs.) (lb.) Itgal (lb.) (lb.) (gal.) (percent) Picklinc Solution-Bath of Invention Series l:

1. AISI 4G19 18.6 30 0` 024 1. 566 0. 042 0.015 0. I3 45. 4 2. AISI lBOH 13.6 l 0. 022 1.588 0. 046 0.013 0.13 33. 3 3. AISI 4020.... 22.4 0.033 1.618 0.079 0.024 0.26 50.0 4. AISI 4620.... 20. 4 35 0. 044 1. 435 0. 062 (l 035 0.13 (l) Total 75. 0 0. 123 0. 229 0. 087 0. 65

Uninhibited Sulfur-ic Acid Baths Series Il 1. AISI 4619 20.3 (l 271 1. 466 0.611 0. 205 0.26 2. AISI 51B60H 14.3 15 0.044 1.433 0. 099 0.037 0.26 3. AISI 4620. 20.5 0.426 l. 444 0. 66S 0. 388 0.40 4. AISI 4620. 1B. 2 35 0. 293 1. 438 0. 500 0.212 0.40 (l) Total 73. 3 l. 034 l. 947 0. 842 1.32

l Each pickled for same length of time: about pickled to substantially complete scale removal.

In Series l, Tests 1-4 were made in the same bath, starting with 3.17 gallons of pickling solution in Test l. In Tests 1-3, no addition was made to the original bath volume of 3.17 gallons, while in Test 4, the bath was brought back initially to the original volume of 3.17 gallons.

In Series II, Tests 1 and 2 were made in one bath, while Tests 3 and 4 were made in a second bath. In each test in Series Il, the bath was brought initially to the same volume (3.17 gal.) and acid content.

As will be seen from the table, the bath of this invention eects a rapid descaling of alloys, with a low acid consumption and a low weight loss of the pickled product.

One of the outslanding features of this bath is its high ferrous iron tolerance. Referring to the drawing, curve A represents the ferrous iron (Fe++) content in grams per liter of one pickling tank for a production run of one week. Curve B represents the acid content range in grams per liter.

Starting with a clean tank on the first operating turn of the rst day of the run, with a pickling bath of 4000 ga`lons of water, 250 gallons of sulfuric acid (56 B.), 20 pounds of 1,3-dibutylthiourea and 5 gallons of 27% aqueous solution of sodium tetradecyl sulfate, the acid was maintained at between 1% and 6% by weight (as 103% H2504). Some siphoning of pickling solution was necessary at intervals to maintain a proper volume of solution in the tank, but ferrous iron was permitted to build up steadily. After four full days of operation, the iron level had reached 200 grams =per liter, at which point 691 tons of steel had been pickled. The bath was run for approximately three more days with the ferrous iron content above 150 g./l., during which time 394 additional tons of steel were pickled. As will be observed from the specific gravity values shown in conjunction with the graph, the gravity of the bath ranged between 1.32 and 1.38 when the iron was maintained at 150 g./l. or above, and at 200 g./l. iron, for example, the iron content of the bath was approximately 14.5%, or approximately 39.5% FeSO4. All specific gravities were determined at room temperature.

The run was discontinued at the end of a week to permit cleaning. Steels pickled included those designated as AISI 4037, 4145, 4340, 1552100, 8640, 4815H, 4150H, 8620H, 01, A4, W2, 55, L6 and P6, -all of which presented clean surfaces when removed from the pickling operation.

Thiourea, or any of the substituted thioureas, may be used in the bath, although the symmetrical thoureas of the group including, for example, diethyl, diisopropyl, dibutyland ditolylthiourea are preferred for the pickling of alloy steels, with 1,3dibutylthiourea producing the best results.

The surfactant ,may be any available alkali metal sec`1 ondary alcohol sulfate in which the alcohol radical has from 8 to 19 carbon atoms. Examples of compounds within this group are 2-ethyl hexyl sulfate, sodium hexaof scale removed in uninhihited sulfuric acid. Each ot the other specimens were decyl sulfate, sodium tetradecyl sulfate, sodium heptau decyl sulfate and sodium undecyl sulfate. Other surfactants of this group are disclosed in U.S. Patent No. 2,088,- 020 to Wickert. It is preferable, when usirig compounds of between l2 and 19 carbon atoms, to have the sulfate radical attached to a carbon atom near the center of the alkyl chain. Compounds in the lower range of carbons,

i.e. those from 8 to 1l carbons, have some tendency to foam.

While the mechanism by which this pickling bath operates is not fully understood, it is recognized that the effect of ferrous iron in the bath is suppressed. There is littfe foaming, especially when surfactants in the higher carbon ranges of the group disclosed are used. The bath pickles a variety of alloy and carbon steels rapidly and cleanly, with no special control problems.

I claim:

1. A pickling method for removing heavy mill scale from an alloy steel article having a coating of mill scale which comprises immersing said article in an aqueous solution in which the active pickling solute consists essentially of sulfuric acid, calculated as H2504, in an amount of from 1% to 20% by weight, a thiourea in an amount of from 0.01% to 0.1% by weight, and an alkali metal monohydric hydrocarbon secondary alcohol sulfate in which the alcohol radical has from 8 to 19 carbon atoms in an amount of from 0.03%f'to 0.2% by weight for a time suicient to remove substantially all of the scale from said article, said solution containing from zero up to grams/liter dissolved iron during pickling.

2. The method according to claim 1 in which sulfuric acid is present in an amount of from 2% to 8%, the thiourea is present in an amount of from 0.01 to 0.05%, and the alkali metal alcohol sulfate is present in an amount of from 0.05% to 0.12%.

3. The method according to claim 1 in which the thiourea is 1,3-dibutylthiourea.

4. The method according to claim 1 in which the sulfate is sodium tetradecyl sulfate.

5. The ,method according to claim 3 in which the sulfate is sodium tetradecyl sulfate.

References Cited UNITED STATES PATENTS 2,392,404 l/ 1946 Peterson 252-149 3,188,292 6/ 1965 Pirotte 252-149 3,312,625 4/1967 Peterson 252-149 OTHER REFERENCES Tergitol Surfactants, Union Carbide, 1961, page 19.

MAYER WEINBLATT, Primary Examiner.

U.S` Cl. X.R. 252-149, 152,' 134-3, 41; 156-18; 148-624; 117-49 

